The present invention relates generally to time domain measurement instruments and more particularly to a time domain measurement instrument having a frequency domain analysis system.
Time domain measurement instruments, such as digital oscilloscopes, use Fast Fourier Transform (FFT) algorithms to generate frequency domain measurements from digital data samples of a time acquired waveform record of an input signal. Sample rate and record length are two important time base parameters that need to be correctly set to produce an accurate frequency domain measurement. The sample rate is set using a horizontal scale knob on the front panel of the oscilloscope. The record length is set by calling up an on-screen horizontal menu on the display device of the instrument. In order to get the desired frequency span and resolution bandwidth requires a user to know about FFTs and how changes to the sample rate and record length oscilloscope parameters affect the frequency span and resolution bandwidth frequency domain measurement. For example, the sample rate and record length are adjusted to acquire a waveform record of an input signal that displays one period of the input signal. The frequency domain waveform record from the FFT has a frequency span and resolution bandwidth defined by the time domain record. Increasing the sample rate increases the maximum frequency domain bandwidth while increasing the record length increases the resolution bandwidth. In the above example, increasing the sample rate without increasing the record length produces a time domain waveform record containing less than a full period of the input signal. Increasing the record length while leaving the sample rate unchanged produces a time domain waveform record containing more than one period of the input signal. To produce a time domain waveform record with one period of the input signal at a higher sample rate requires changing both the sample rate and the record length. This requires the user to individually change the sample rate using the horizontal scale knob and the record length parameter in the horizontal menu. Such a process is inefficient and time consuming.
The digital data samples of the time domain waveform record are processed by the FFT algorithm and displayed in a frequency versus magnitude plot on the display device of the measurement instrument. As was described above, the frequency span and the resolution bandwidth of the frequency domain display can only be changed by returning to the time domain set-up and changing the sample rate and frequency span.
An improvement to the above described frequency domain processing of time base data is to add a windowing function as part of the FFT process. The windowing function may be any number of well known functions, such as Gausian, Kaiser Bessel, Hanning or the like. The window function is applied to the acquired waveform record samples and processed by the FFT. Frequency span and center frequency controls have been added to time domain measurement instruments but they have not been integrated with the time base controls. A drawback to this design is that adjusting the center frequency past the region covered by the FFT results in a frequency domain data being moved off screen. A user must remember in which direction to adjust the center frequency control to get the frequency domain display back on the screen. It is possible for the user to lose track of the frequency spectrum data thus requiring resetting of the time base parameters.
What is needed is a frequency domain analysis system for a time domain measurement instrument having an integrated time base and frequency domain controls. The time base controls should provide individual controls for the sample rate and record length but also be able to concurrently control both parameter settings. The system should also provide a bridge between the time domain acquired waveform record and the frequency domain data generated by the spectral analysis system. The system should provide displays of frequency versus magnitude and frequency versus phase.
Accordingly, the present invention is to a frequency domain analysis system incorporated into a time domain measurement instrument having an acquisition system that generates a waveform record of digital data samples of an input signal. The frequency domain analysis system has a duration control that adjusts acquisition time intervals of a waveform record in seconds and a resolution control that adjusts the number of digital data samples over a specified duration. The duration control has a time control that adjusts the length of the waveform record in seconds, a sample rate control that adjusts the acquisition system sample rate, and a record length control that adjusts the number of samples in the waveform record. The resolution control has a sample interval control that adjusts the time interval between digital data samples in the waveform record by concurrently changing the sample rate and the record length of the acquired waveform record while maintaining the length of the waveform record constant.
The acquired waveform record is applied to a frequency spectrum gate having controls that adjust a gate duration in seconds and a gate length in samples and positions the gate over the digital data samples of the waveform record. The digital data samples of the waveform record within the gate are applied to a window filter. A spectrum analysis generator, having center frequency, frequency span and resolution bandwidth controls, receives the filtered waveform record within the gate and generates frequency domain values over the gated waveform record. The spectrum analysis generator outputs frequency domain values defined by the frequency span, center frequency and resolution bandwidth controls. A coordinate transformer receives the complex frequency domain values and generates magnitude and phase values defined by polar coordinates.
The time domain measurement instrument has trigger circuitry that generates a trigger pulse for generating the waveform record of digital data samples. The position control of frequency spectrum gate adjusts the center position of the gate in relation to the trigger pulse. The gate duration controls of the frequency spectrum gate further adjusts the resolution bandwidth of the spectrum analysis generator and the resolution bandwidth control of the spectrum analysis generator further adjusts the gate duration of the frequency spectrum gate. In the preferred embodiment, the spectrum analysis generator further comprises a zero-fill Fast Fourier Transform.